I work for a development charity and put in a lot of miles across the Atlantic and around Africa and I've never flown anything other than economy. As far as I can tell, for those of us at the back end of the plane, there is very little to choose between the major airlines. The seat pitch is always too small (and it seems to get smaller the longer you are in the air), the food is acceptable and the staff are friendly and efficient. KLM are no better or worse than the others.
That being said, one of our best family memories come from a flight with KLM. The crew discovered that we were leaving West Africa after twelve years in the heat and humidity and welcomed us aboard with champagne and cokes for the kids even though we were in economy. Just a little gesture, but it did us an awful lot of good.